Difference's in material's is what drives convection.

temperature and density

There are three types of heat transfer--radiation, conduction, and convection and the type where the heat is transferred and characterized by a circular movement of currents within a fluid is called convection.  Differences in material's temperature and density is what drives convection. Convection currents are formed by the heating and cooling of the fluid (temperature change),  changes in the fluid's density or mass, and the force of gravity combine to set the convection currents in motion.

the correct answer would be 6 feet.